Many existing short-range optical fiber systems utilize multi-mode fiber (MMF). Multi-mode fiber has been widely used because of its low price, ease of installation, and its specification within several standards, such as the Fiber Data Distribution Interface (FDDI). However, MMF suffers from low bandwidth, and thus can only support transmission at relatively low speeds (tens to hundreds of megabits per second). As 10 Gigabit Ethernet (10 GbE) is increasingly introduced into optical communications networks, the bandwidth limitation of the existing MMF links becomes a new challenge for a network designer to achieve a meaningful transmission distance. The numerical product of bandwidth multiplied by distance, is a useful measure of the data carrying capacity of optical fiber. For example, 62.5 □m MMF has only a 500 Mhz-km bandwidth-distance product for transmission of light signals at 1310 nm and only 160 MHz-km bandwidth-distance product for transmission of light signals at 850 nm. Thus, for 10 Gbit/s Ethernet (10 GbE) applications, the useful propagation distance through MMF is only about 80 meters at 1310 nm and 26 meters at 850 nm.
Modal dispersion is the principal bandwidth-limiting factor in MMF. Light propagates down the fiber core in a stable “path,” known as a “mode.” Multi-mode fiber supports hundreds of modes in the core, each of which is a different length. An example of light propagation within MMF is shown in FIG. 1. The known multi-mode fiber 100 shown in FIG. 1 comprises a core 104 surrounded by a single cladding 102. The diameter of the core 104 is sufficiently great that multiple transmission modes can propagate along the MMF. Four such modes, 106, 108, 110 and 112, are schematically shown in FIG. 1. If one launches a single pulse of light into the MMF 100, the light will excite and will be transmitted by the various modes, 106–112 and others, all of such modes reflecting, internally within the MMF, at different angles off the core/cladding interface. In other words, the light in each mode will travel a different distance depending on the modal path, so the light in some modes will arrive at the far end of the fiber later than others. For instance, as schematically shown in FIG. 1, light propagating in mode 106 takes the shortest path through the MMF 100 and light propagating in mode 112 takes the longest path through the MMF 100. This phenomenon is known as modal dispersion. If multiple pulses are launched into the fiber 100, they will all suffer such modal dispersion. As a result, adjacent pulses may overlap after a certain transmission distance such that the receiver cannot distinguish one pulse from another, introducing bit errors. In a sharp contrast, single-mode fiber (SMF), used in long-distance fiber-optic communications systems, eliminates the modal dispersion effect because only the propagation of one (fundamental) mode is supported within SMF. This makes SMF work very well for long-distance communications.
Various prior-art solutions have been proposed to extend MMF transmission distance at high data rates. These are briefly discussed in the following paragraphs.
1. Deployment of new fiber. A new generation of so-called “laser optimized multi-mode fiber” has been developed to replace the existing conventional MMF. However, since replacing existing MMF is usually quite expensive, many enterprise customers decide to continue using the existing legacy MMF instead of switching to the new generation fiber.
2. Electrical solution—Electronic Dispersion Compensation. Electronic dispersion compensation (EDC) can be accomplished by a semiconductor chip that performs blind post-detection adaptive equalization on the electrical signal output from the optical detector. This method is still under development and so far only works under certain limited conditions. It also adds complexity and cost to the design of detectors and transceivers deployed in a system using MMF.
3. Optical solution—Restricted Mode Launch. The technique of Restricted Mode Launch (RML), as opposed to overfilled launch (OFL, i.e., uniform excitation of all modes) has been used to minimize modal dispersion in systems employing MMF, by introducing light into only a certain sub-set of all the modes. In a first example of the prior-art RML technique, a light signal is launched into either the fundamental mode or into a limited small set of lowest-order modes to mitigate the modal dispersion. A prior-art system 200 using this technique is schematically shown in FIG. 2. The prior-art system 200 comprises a transmitter 214 producing a light signal, a short section of single-mode fiber (SMF) 202 receiving the light signal from the transmitter 214, a section of multi-mode fiber (MMF) 204 receiving the light signal from the SMF 202 and a receiver 216 at the opposite end of the MMF 204 and receiving the light signal from the MMF 204. The SMF 202 comprises a core 208 surrounded by a cladding 206. The MMF 204 comprises a core 212 surrounded by a cladding 210. In the system 200, restricted mode launch is achieved by coaxially aligning the SMF 202 to the MMF 204 such that axes of both the SMF 202 and MMF 204 coincide along axis 201 and such that the SMF core 208 is optically coupled to the center portion 203 of the MMF core 210. This method is known as a center launch (CL) method. Unfortunately, if there should exist, in the fiber refractive index profile, a localized distinct peak or dip at the core center, which is normal in legacy MMF, the lowest order modes may have largely different propagation times when compared with higher order modes. This results in increased mode dispersion and causes link failure.
In a second example of the RML technique, light is launched into a small number of higher order modes to minimize the modal dispersion and, thus, to increase the transmission distance. This technique is described in the paper by M. Webster, et al., “A statistical analysis of conditioned launch for gigabit Ethernet links using multimode fiber,” J. of Lightwave Technology, pp. 1532, vol. 17, no. 9, 1999. Typically, a mode conditioning patch cord, is used to excite higher-order modes through the technique of offset launch (OSL). Unfortunately, this technique only works for data transmission rates that are less than 1 to 2 Gbit/s.
4. Optical solution—center launch and mode filtering upon reception. Restricted mode launch alone (described above) is not an ultimate solution as it only works under certain conditions of uniform modal dispersion, whereas the modal dispersion along the legacy fiber is highly unpredictable. The modal dispersion can be reduced to virtually zero, however, if the receiver is permitted to detect only one mode. A prior-art system 300 employing this technique is shown in FIG. 3. The prior-art system 300 comprises a transmitter 214 producing a light signal 301, a first short section of single-mode fiber (SMF) 202 receiving the light signal 301 from the transmitter 214, a section of multi-mode fiber (MMF) 204 receiving the light signal 301 from the SMF 202, a second short section of SMF 202b at the opposite end of the MMF 204 and receiving the light signal 301 from the MMF 204 and a receiver 216 receiving the light signal 301 from the SMF 202b. The SMF 202 comprises a core 208 surrounded by a cladding 206. The MMF 204 comprises a core 212 surrounded by a cladding 210. The second SMF 202b comprises a core 208b surrounded by cladding 206b. As shown in FIG. 3, with a center launch condition, the second SMF 202b is used to filter out higher order modes before the light hits the receiver 216. In this case, the receiver will detect only the fundamental mode. The SMF-MMF alignment is achieved with a mechanical splice. Although the center launch scheme combined with mode filtering (FIG. 3) introduces little loss of the optical signal at the transmitter side, high attenuation is often observed at the receiving splice due to the mode field diameter mismatch from MMF to SMF.
The bandwidth-distance product of a MMF system is increased by selectively propagating only a limited number of modes through the MMF link. Ideally, the modal dispersion is eliminated if only one mode propagates. A SMF, as implied by its name, can filter out all higher order modes of the MMF when centrally coupled to the MMF, and allows only the fundamental mode to reach the detector. However, due to the mismatch of the core diameters, it is expected that direct coupling from a MMF to a SMF will result in high attenuation. The attenuation can be estimated by
      IL    ⁡          (      dB      )        =            -      10        ⁢          log      ⁡              [                              (                                          ⅅ                2                                            ⅅ                1                                      )                    2                ]            where D1 and D2 are the core diameters of the MMF and the SMF, respectively. For example, an over-filled (uniform excitation of all modes) 62.5 μm (D1) MMF and a 9 μm (D2) SMF cause an insertion loss as high as 16.8 dB.
The insertion loss can be made lower than described by the above equation under the condition of center launch with a SMF. Under this condition, the main contribution to the attenuation is the mismatch of the mode field diameter of the fundamental modes in the two fibers, and can be estimated by
      IL    ⁡          (      dB      )        =            -      10        ⁢          log      [              4                              (                                                            ω                  2                                                  ω                  1                                            +                                                ω                  1                                                  ω                  2                                                      )                    2                    ]      where ω1,2 is the mode field diameters of the fundamental modes in two fibers, respectively. In a technical article by Z Haas, and M. A. Santoro, titled “A mode filtering scheme for improvement of the bandwidth-distance product in multimode fiber system,” (J. of Lightwave Technology, pp. 1125, vol. 11, no. 7, 1993), an insertion loss of approximately 5–6 dB was observed after 2 km transmission. We have observed about 2–3 dB attenuation after a MMF link of 300 m (the target distance for 10 Gbit/s Ethernet applications).
Obviously, MMF to SMF coupling loss translates directly into power penalty, and demands a larger power budget in the design of a transmission system using MMF.
In view of the above-described difficulties of prior-art techniques for using multi-mode fiber at both high bit rates and relatively long distances, there is a need in the art for an improved system and method for a mode filtering optical coupler for multimode fiber-optic transmission systems. The present invention addresses such a need.